Thixotropic polysiloxane pastes for additive manufacturing

ABSTRACT

Shelf-stable, rapid crosslinking, “all-in-one” pastes useful as “inks” in additive manufacturing are provided. These pastes exhibit desirable rheological flow properties and crosslinking upon exposure to UV light. The pastes are based on vinylsilyl-functionalized, completely amorphous, linear terpolysiloxanes containing predominantly dimethylsiloxy-repeat units with small amounts of diphenylsiloxy-, methylphenylsiloxy-, diethylsiloxy-, and/or methyltrifluoroalkylsiloxy-crystallization disruptors. The base polymers are preferably compounded with a trimethylsilylated-hydrophobic silica filler, thixotropic flow agent, hydrosilyl-functionalized oligomeric crosslinker, and a catalytic system comprising platinum(II) acetylacetonate or trimethyl(methylcyclopentadienyl)-platinum(IV), and diethyl azodicarboxylate.

RELATED APPLICATIONS

The present application is a continuation application of pending U.S.patent application Ser. No. 17/192,197, filed Mar. 4, 2021, entitledTHIXOTROPIC POLYSILOXANE PASTES FOR ADDITIVE MANUFACTURING, incorporatedby reference in its entirety herein. The '197 application claims thepriority benefit of U.S. Provisional Patent Application Ser. No.63/147,985 filed Feb. 10, 2021, entitled THIXOTROPIC POLYSILOXANE PASTESFOR ADDITIVE MANUFACTURING, incorporated by reference in its entiretyherein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No.DE-NA-0002839, awarded by the United States Department ofEnergy/National Nuclear Security Administration. The Government hascertain rights in the invention.

BACKGROUND Field

The present disclosure relates to compositions useful in additivemanufacturing processes.

Description of Related Art

Additive manufacturing, also referred to as 3D printing, has become apopular technological platform for computer-assisted design and rapidmanufacturing. Among various printing techniques developed, extrusionhas become increasingly popular because of easily available andrelatively inexpensive equipment as well rather simple operatingprocedures. Of the polymers for use as printing “inks” inextrusion-based additive manufacturing, thermoplastics that convenientlymelt above room temperature and quickly solidify upon cooling to roomtemperature still dominate the field, although elastomers for thispurpose have started to find use more recently. Among the latter,various types of polysiloxanes have attracted attention because of thethermal, surface, biomedical, and permeation properties of siliconerubbers.

The difficulty with elastomers for additive manufacturing, however,arises from their characteristic low glass and melting temperatures,which create the need to suitably combine two main processingrequirements: an appropriate rheology for the material's flow through aprinter/extruder; and quick, efficient, and permanent fixing of theshape of the printed object. In practice, this can be achieved by: (a)formulating elastomer “inks” into thixotropic pastes that flow likeliquids above certain yield stress to which they are exposed in theprinter/extruder but assume and retain (at least temporarily) theirshapes after the stress is released, and (b) designing the compositionso that it can be quickly and efficiently covalently crosslinked (curedwithout support) in air after the deposition from an extruder'sdie/nozzle.

For the latter, hydrosilylation crosslinking chemistry is appealingbecause it can be controlled, is reproducible, and does not incorporateinto the crosslinked polysiloxane any additional structural units thatmay become weak links capable of disturbing properties of the resultingrubbers in both low and high temperature applications. A problem withhydrosilylation, however, is that when the crosslinking components aremixed together with necessary catalyst(s), the reaction readilycommences and progresses relatively quickly. As a result, theseformulations must be produced as two-part systems in which the catalystis dispersed in only one of the crosslinking components, with the twoparts being mixed together in appropriate proportions at the applicationlocation immediately prior to use, resulting in curing mixtures withvery limited, short lifetimes. This clearly represents a problem forprinters, where having “inks” with long shelf lives, ready to go whendesired without the need for any additional manipulations (such asaddition of other chemicals at the location) would be highly preferable.

Thus, there is a need for an “all-in-one” composition that has a longshelf life and provides convenient rheological flow through the extruderas well as quick, reliable, and permanent 3D crosslinking of printedshapes after exiting the extruder die.

SUMMARY

In one embodiment, the present disclosure is broadly concerned with amethod of forming a three-dimensional structure, where the methodcomprising one or more of the following:

-   (i) curing a first composition to form a first layer, with the first    composition comprising at least 50% by weight of a polymer that    comprises the following monomers:

and

-   -   a crystallization disruptor monomer comprising:

-   -   where:        -   each R₁ can be the same or different and is chosen from            alkyls and fluoroalkyls;        -   R₂ is chosen from alkyls; and        -   each R₃ can be the same or different and is chosen from            phenyl, alkyls, and fluoroalkyls, wherein at least one R₃ is            phenyl, ethyl, or a fluoroalkyl;

-   (ii) curing a second composition to form a second layer on the first    layer, wherein the first and second compositions can be the same as    or different from one another; and

-   (iii) repeating (ii) one or more times with further compositions    that can be the same as the first composition or different from the    first composition so as to form one or more additional layers,    wherein (ii) or (iii) results in the formation of the    three-dimensional structure.

The disclosure is also concerned with the three-dimensional structuresformed by the above method.

Finally, the disclosure provides a composition useful in additivemanufacturing methods. The composition comprises:

at least 50% by weight of a polymer that comprises the followingmonomers:

and

-   -   a crystallization disruptor monomer comprising:

-   -   where:        -   each R₁ can be the same or different and is chosen from            alkyls and fluoroalkyls;        -   R₂ is chosen from alkyls; and        -   each R₃ can be the same or different and is chosen from            phenyl, alkyls, and fluoroalkyls, wherein at least one R₃ is            phenyl, ethyl, or a fluoralkyl;

a crosslinker;

a thixotropic additive; and

one or more of a catalyst, a catalyst inhibitor, or a filler.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the oscillatory shear of pastes containing 15 wt. %AEROSIL® R 812 S in MePhS-containing terpolymer (B) and varying amountsof BLUESIL™ THIXO ADD 22646 (“THX”) as described in Example 5;

FIG. 2 is a graph depicting the steady shear of pastes containing 15 wt.% AEROSIL® R 812 S in MePhS-containing terpolymer (B) and varyingamounts of BLUESIL™ THIXO ADD 22646 (“THX”) as described in Example 5;

FIG. 3 is a graph showing the steady shear of pastes prepared fromDiPhS-, MePhS- and DiEtS-containing terpolymers (A, B, and C) at day 0with 15 wt. % AEROSIL® R 812 S and 1 wt. % BLUESIL™ THIXO ADD 22646(“THX”) as compared to a commercially available product (“SE 1700”) asdescribed in Example 6;

FIG. 4 is a graph of the oscillatory shear of pastes prepared fromDiPhS-, MePhS- and DiEtS-containing terpolymers (A, B, and C) at day 0with 15 wt. % AEROSIL® R 812 S and 1 wt. % BLUESIL™ THIXO ADD 22646(“THX”) as compared to a commercially available product (“SE 1700”) asdescribed in Example 6;

FIG. 5(A) is a graph depicting the oscillatory shear of pastes preparedfrom DiPhS-containing terpolymer (A) at day 0 with 10, 15, and 18 wt. %of AEROSIL® R 812 S and 1 wt. % of BLUESIL™ THIXO ADD 22646 (“THX”) ascompared to a commercially available product (“SE 1700”) as described inExample 7;

FIG. 5(B) is a graph showing the oscillatory shear of “all-in-one”pastes prepared from MePhS-containing terpolymer (B) at day 0 with 10,15, and 18 wt. % of AEROSIL® R 812 S and 1 wt. % of BLUESIL™ THIXO ADD22646 (“THX”) as compared to a commercially available product (“SE1700”) as described in Example 7;

FIG. 5(C) is graph of the oscillatory shear of pastes prepared fromDiEtS-containing terpolymer (C) at day 0 with 10, 15, and 18 wt. % ofAEROSIL® R 812 S and 1 wt. % of BLUESIL™ THIXO ADD 22646 (“THX”) ascompared to a commercially available product (“SE 1700”) as described inExample 7;

FIG. 6(A) is a graph depicting the steady shear of pastes prepared fromDiPhS-containing terpolymer (A) at day 0 with 10, 15, and 18 wt. %AEROSIL® R 812 S and 1 wt. % BLUESIL™ THIXO ADD 22646 (“THX”) ascompared to a commercially available product (“SE 1700”) as described inExample 7;

FIG. 6(B) is a graph showing the steady shear of pastes prepared fromMePhS-containing terpolymer (B) at day 0 with 10, 15, and 18 wt. %AEROSIL® R 812 S and 1 wt. % BLUESIL™ THIXO ADD 22646 (“THX”) ascompared to a commercially available product (“SE 1700”) as described inExample 7;

FIG. 6(C) is a graph of the steady shear of pastes prepared fromDiEtS-containing terpolymer (C) at day 0 with 10, 15, and 18 wt. %AEROSIL® R 812 S and 1 wt. % BLUESIL™ THIXO ADD 22646 (“THX”) ascompared to a commercially available product (“SE 1700”) as described inExample 7;

FIG. 7 is a graph comparing cure times of films from pastes made fromMePhS-containing terpolymer (B) with 15 wt. % AEROSIL® R 812 S and 1 wt.% BLUESIL™ THIXO ADD 22646 as a function of MeCpPtMe₃ concentration and[DEAD]:[catalyst] ratio as described in Example 8;

FIG. 8 is a graph comparing cure times of films from pastes made fromMePhS-containing terpolymer (B) with 15 wt. % AEROSIL® R 812 S and 1 wt.% BLUESIL™ THIXO ADD 22646 as a function of Pt(AcAc)₂ concentration and[DEAD]:[catalyst] ratio as described in Example 8;

FIG. 9(A) is a photograph of a sample during the DIW printing processdescribed in Example 9;

FIG. 9(B) is a photograph of the silicone rubber pad printed in Example9;

FIG. 9(C) demonstrates the flexibility of the rubber pad shown in FIG.9(B); and

FIG. 9(D) is a cross-sectional view (120×) of the printed rubber pad ofFIG. 9(C).

DETAILED DESCRIPTION

The present disclosure provides polysiloxane compositions and methods ofusing those compositions in additive manufacturing. These compositionsare preferably thixotropic, meaning they exhibit a decrease in viscositywith increasing shear, and the viscosity will return as the sheardecreases.

Polysiloxane Compositions 1. Polymers for Use in Compositions

Polymers for use in the compositions are preferablyvinylsilyl-functionalized polysiloxanes. The polymers are preferablylinear, although they can be branched in some embodiments, or even amixture of branched and linear. The polymers are also preferablysubstantially or completely amorphous, telechelic, and/ordimethylvinylsilyl-terminated.

In a preferred embodiment, the polymer comprises the following repeatunits:

and

-   -   a crystallization disruptor monomer comprising:

where:

-   -   each R₁ can be the same or different and is chosen from alkyls        (preferably C₁ to about C₁₄, more preferably C₁ to about C₆ and        even more preferably C₁ to about C₃), fluoroalkyls (preferably        C₁ to about C₁₄, more preferably C₁ to about C₆, and even more        preferably C₁ to about C₃), and hydroxyl;    -   R₂ is chosen from alkyls (preferably C₁ to about C₁₄, more        preferably C₁ to about C₆, and even more preferably C₁ to about        C₃); and    -   each R₃ can be the same or different and is chosen from phenyl,        alkyls (preferably C₁ to about C₁₄, more preferably C₁ to about        C₆, and even more preferably C₁ to about C₃), and fluoroalkyls        (preferably C₁ to about C₁₄, more preferably C₁ to about C₆, and        even more preferably C₁ to about C₃), wherein at least one R₃ is        phenyl, ethyl, or a fluoralkyl.

In one embodiment, each R₁ and R₂ is individually chosen from alkyls,while each R₃ is individually chosen from phenyl, alkyls, or afluoralkyl. In a particularly preferred embodiment, each R₁ and R₂ ismethyl (i.e., dimethylsiloxy and methylvinylsiloxy monomers,respectively), and (III) is chosen from one or more of:

In one embodiment, the molar percentage of (I) in the polysiloxane ispreferably from about 90% to about 98%, more preferably from about 93%to about 96%, and most preferably from about 95% to about 96%. The molarpercentage of (II) is from about 0.1% to about 10%, preferably fromabout 0.1% to about 5%, more preferably from about 0.1% to about 2%, andeven more preferably from about 0.3% to about 1%. The molar percentageof (III) is from about 2% to about 60%, preferably from about 2% toabout 10%, more preferably from about 3% to about 9%, and even morepreferably from about 3% to about 7%.

In embodiments where (III) is diphenylsiloxy, (III) is preferablypresent in the polymer at a molar percentage of about 1.5% to about 6%,more preferably about 2.5% to about 5%, and more preferably about 3.3%to about 4%. In embodiments where (III) is methylphenylsiloxy, (III) ispreferably present in the polymer at a molar percentage of about 2% toabout 8%, more preferably about 3% to about 6%, and more preferablyabout 4% to about 5%. In embodiments where (III) is diethylsiloxy, (III)is preferably present in the polymer at a molar percentage of about 3%to about 11%, more preferably about 5% to about 9%, and more preferablyabout 6.5% to about 7.5%.

In one embodiment, the polysiloxane consists essentially of (I), (II),and (III). In another embodiment, the polysiloxane consists of (I),(II), and (III) (i.e., the polysiloxane is a terpolymer).

The number average molecular weight (Mn) of the resulting polymer ispreferably from about 10,000 g/mol to about 70,000 g/mol, morepreferably from about 25,000 g/mol to about 45,000 g/mol, and even morepreferably from about 28,000 g/mol to about 36,000 g/mol. Theweight-average molecular weight (Mw) range of the polymer as measured bySEC-MALS-VIS (using a Visco-Star II online detector) is from about25,000 g/mol to about 85,000 g/mol, more preferably from about is 40,000g/mol to about 65,000 g/mol, and even more preferably from about 45,000g/mol to about 60,000 g/mol.

2. Polymerization Materials and Methods

The above-described terpolymers can be prepared according to knownpolymerization methods. One preferred method comprises anionic ringopening/equilibration polymerization of the corresponding mixtures ofcyclosiloxanes, as carried out in Example 2 below and as described bythe following, each incorporated by reference:

-   -   i. A. Zlatanic, D. Radojcic, X. Wan, J. M Messman, P. R.        Dvornic, Suppression of Crystallization in Polydimethylsiloxanes        and Chain Branching in Their Phenyl-containing Copolymers,        Macromol. 50 (2017) 3532-3543;    -   ii. A. Zlatanic, D. Radojcic, X. Wan, J. M. Messman, P. R.        Dvornic, Monitoring of the Course of the Silanolate-Initiated        Polymerization of Cyclic Siloxanes. A Mechanism for the        Copolymerization of Dimethyl and Diphenyl Monomers, Macromol.        51 (2018) 895-905; and    -   iii. A. Zlatanic, D. Radojcic, X. Wan, J. M. Messman, D. E.        Bowen, P. R. Dvornic, Dimethyl-Methylphenyl Copolysiloxanes by        Dimethyl silanolate-Initiated Ring Opening Polymerization.        Evidence for Linearity of the Resulting Polymer Structures. J.        Pol. Sci., Part A: Pol. Chem. 57(10) (2019) 1122-1129,        https://doi.org/1.002/pola.29367.

3. Composition Preparation

The compositions according to this disclosure are prepared by mixing thepolymer described above with the other components until substantiallyuniformly mixed. Alternatively, the composition can be prepared bymixing all ingredients except the filler, followed by successiveaddition and mixing of small portions of filler until all filler hasbeen added, followed by further mixing until substantially uniformlymixed. Advantageously, because the compositions are shelf-stable, thiscomposition can be provided as an “all-in-one” composition rather than atwo-part system whose two parts must be stored separately and then mixedimmediately at use.

The composition comprises one or more of the following a crosslinker, athixotropic additive, a catalyst, a catalyst inhibitor, and/or a filler.The above-described polysiloxane will preferably be present in thecomposition at a level of at least about 50% by weight, and morepreferably about 50% to about 75% by weight, based upon the total weightof the composition taken as 100% by weight.

Preferred crosslinkers are hydrosilylation crosslinkers, with preferredsuch crosslinkers including a methylhydridosiloxane-dimethylsiloxanecopolymer. Examples of other suitable hydrosilylation crosslinkersinclude those chosen from HMS-151 (15-18% methylhydridosiloxane andtrimethylsiloxane terminated; by Gelest, Inc.), HMS-301 (25-35%methylhydridosiloxane and trimethylsiloxane terminated; by Gelest,Inc.), HMS-H271 (25-30% methylhydridosiloxane and hydride terminated; byGelest, Inc.), and mixtures thereof.

In one embodiment, the polysiloxane and crosslinker are provided atlevels that correspond to a [Si—H]:[Si-Vi] molar ratio of from about0.5:1 to about 1.5:0, preferably from about 0.5:1 to about 1.5:0.1, morepreferably from about 0.5:1 to about 1.5:1, and even more preferablyabout 1:1.

Preferred thixotropic additives include those having apolydimethylsiloxane main chain backbone and poly(ethyl ether-co-propylether) pendant chains ending in carbinol end groups, methylether endgroups, or both. One thixotropic additive meeting this description issold under the name BLUESIL™ THIXO ADD 22646 (from Elkem Silicones).Additional thixotropic additives suitable for use in the compositionsaccording to the disclosure include one or more of Thixo Agent AC (CHT,USA/Quantum Silicones), DMS-H11 (hydride terminatedpolydimethylsiloxane; Gelest, Inc.), PLY-906 (NuSil), and AlumiliteThixotropic Additive (Alumilite Corporation). It is preferred that thethixotropic additive is present in the composition at levels of about0.1% by weight to about 5% by weight, more preferably about 0.5% byweight to about 3% by weight, and even more preferably about 0.5% byweight to about 1.5% by weight, based on the total weight of thecomposition taken as 100% by weight.

Preferred catalysts are hydrosilylation catalysts, with a transitionmetal (e.g., platinum, rhodium, ruthenium, palladium, nickel, iron,iridium) catalyst being particularly preferred. Such catalysts includeplatinum(II) acetylacetonate,trimethyl(methylcyclopentadienyl)-platinum(IV) and other UV-activehydrosilylation catalysts, such as those disclosed by Lukin, Ruslan Yuet al., “Platinum-Catalyzed Hydrosilylation in Polymer Chemistry.”Polymers 12, 2174 (2020), incorporated by reference herein. It ispreferred that the catalyst is present in the composition at levels ofabout 0.0005% by weight to about 1% by weight, more preferably about0.00075% by weight to about 0.5% by weight, and even more preferablyabout 0.001% by weight to about 0.2% by weight, based on the totalweight of the composition taken as 100% by weight.

Although not required, in certain embodiments a catalyst inhibitor isalso included in the composition. Preferred catalyst inhibitors have along pot life and sharp onset cure, and will also complex with thecatalyst to successfully inhibit the catalyst at room temperature duringmolding or storage. Alkynes and alkenes with electron-withdrawingsubstituents are particularly preferred for use as the catalystinhibitor. Maleates and fumarates are particularly well-suited for thisrole. Examples include those chosen from diethyl azodicarboxylate,1-ethynylcyclohexanol dimethyl maleate, diallyl maleate, dimethylmaleate, and dimethyl fumarate. Other suitable inhibitors are discussedby Lukin, Ruslan Yu et al., “Platinum-Catalyzed Hydrosilylation inPolymer Chemistry.” Polymers 12, 2174 (2020), previously incorporated byreference.

It is preferred that the catalyst inhibitor is present in thecomposition at levels of about 0% by weight to about 1.5% by weight,more preferably from about 0.01% by weight to about 1.5% by weight, evenmore preferably about 0.02% by weight to about 1.0% by weight, and mostpreferably about 0.05% by weight to about 0.75% by weight, based on thetotal weight of the composition taken as 100% by weight.

The catalyst and catalyst inhibitor can be provided individually, orthey can be delivered together as part of a “catalyst system” thatpreferably includes a carrier (e.g., 1,3-dioxolane) for the twocomponents. The molar ratio of catalyst inhibitor to catalyst ispreferably from about 0.25:1 to about 4:1, and more preferably 1:1 toabout 4:1.

Preferred fillers for use in the composition include silica (preferablyfumed), montmorillonite, carbon black, zinc oxide, titanium dioxide,carbon nanotubes, graphene/reduced graphene oxide, and mixtures thereof.A particularly preferred filler is a trimethylsilylated fumed silicasuch as that sold under the name AEROSIL® R 812 S (Evonik, Olmstead,Ohio). It is preferred that the filler is present in the composition atlevels of about 6% by weight to about 24% by weight, more preferablyabout 8% by weight to about 20% by weight, and even more preferablyabout 10% by weight to about 18% by weight, based on the total weight ofthe composition taken as 100% by weight.

Although a solvent is not required in the compositions according to thedisclosure, one could be present (in addition to any carrier for thecatalyst and catalyst inhibitor). In other embodiments, the compositiondoes not include any solvent other than the carrier, and in someinstances does not include any solvent or the carrier. Thus, solventlevels of the composition are preferably less than about 50% by weight,more preferably less than about 25% by weight, even more preferably lessthan about 5% by weight, and even more preferably about 0% by weight,based upon the weight of the composition taken as 100% by weight.

In one embodiment, the composition consists essentially of, or evenconsists of, the polysiloxane, crosslinker, thixotropic additive,catalyst, catalyst inhibitor, and filler.

In another embodiment, it is preferred that the composition compriselittle to no polycarbonates. That is, the composition will comprise lessthan about 10% by weight polycarbonates, preferably less than about 5%by weight polycarbonates, more preferably less than about 1% by weightpolycarbonates, and more preferably about 0% by weight polycarbonates,based on the total weight of the composition taken as 100% by weight.

4. Composition Properties

Regardless of the embodiment, the composition will possess rheologicalproperties (determined as defined in Example 4) making it highly usefulin additive manufacturing, including a high storage modulus and highshear stress. Also, this behavior remains unchanged for a month or moreof storage at room temperature, particularly when stored in the dark.For example, the compositions will preferably have yield stresses ofabout 1,000 Pa to about 3,500 Pa, and preferably about 1,300 Pa to about2,400 Pa (if G′/G″ intersect was used).

Additionally, the paste compositions described herein will rapidlycrosslink when exposed to electromagnetic radiation (e.g., UV light atabout 315 to about 400 nm; IR radiation from about 700 nm to about 1 mm)but will exhibit little to no crosslinking for some amount of time inambient light or dark conditions. For example, upon exposure toelectromagnetic radiation, the compositions will crosslink (i.e., pastetackiness disappears—greater than 95% gel content) within about 200seconds, preferably within about 140 seconds, and more preferably withinabout 120 seconds. Furthermore, in ambient light (e.g., typicallaboratory light), the compositions will show no visible signs ofcrosslinking (e.g., no signs of tackiness upon contact with a tonguedepressor or similar rigid object) for at least about 4 days, preferablyat least about 6 days, and more preferably at least about 10 days.Finally, in a dark environment, the compositions will show no visiblesigns of crosslinking for at least about 30 days, preferably at leastabout 3 months, more preferably at least about 6 months, and even morepreferably at least about 9 months. All of the foregoing is obtained atroom temperature (i.e., about 20° C. to about 25° C.).

Methods of Using the Compositions

The paste compositions described herein can be used in additivemanufacturing, which is a process of generating a three-dimensionalstructure or object by sequential addition of layers of material or byjoining of material layers or parts of material layers to form thosestructures. “3D printing” refers to a type of additive manufacturingwhere the “ink” or composition of which the structure three-dimensionalstructure will be made is deposited using a nozzle or other printertechnology. “3D printer” refers to equipment used for 3D printing.

The disclosed composition is suitable for use in any available additivemanufacturing machine technologies but particularly in those thatbenefit from the use of an elastomeric material, including materialextrusion (where “ink” is dispensed through a nozzle or orifice) and/ormaterial jetting (where droplets or material are deposited to build thelayers). These are often referred to as “Direct Ink Write” (“DIW”)processes.

In embodiments where material extrusion or material jetting is to beutilized, the composition disclosed herein will be dispensed “asreceived” (i.e., no mixing of components or parts right beforedispensing) through a conventional 3D printer nozzle on a printer headthat moves around the area to be printed, under the control anddirection of conventional 3D software. The material is deposited in avery thin layer (e.g., less than about 400 microns, or less than about200 microns, or less than about 100 microns, or less than about 50microns) in the desired pattern for that layer. During this application,electromagnetic radiation, preferably in the form of UV light at awavelength of about 315 nm to about 400 nm, is applied to thecomposition, which initiates crosslinking by hydrosilylation of thepolysiloxane in the composition. As described previously, thiscrosslinking is essentially instantaneous—as quick as about 20-30seconds—under these conditions and does not require a second ordifferent cure step such as heat. In other words, the process is asingle-cure process that can be carried out at room temperature (about20° C. to about 25° C.) and without requiring the nozzle or orifice tobe heated. Additionally, depending on the application, curing can takeplace in the air or on a support, thus a support is not required withthe present process beyond the support on which printing is taking place(e.g., aluminum plate coated with polytetrafluoroethylene; glass plate).

The foregoing layer formation is repeated as many times as necessary andin the pattern necessary to form the desired three-dimensional object.It will be appreciated that each subsequent layer can be formed from thesame composition as the first layer or, in situations where amulti-material object is desired, the composition can be switched asneeded at any layer(s).

Because these polysiloxane compositions include all necessary componentsfor both successful printing and subsequent crosslinking from the momentthey are manufactured, perhaps their most significant advantage is thatthey are completely finalized/compounded at the manufacturing facilityand delivered to the printer ready for use. No additional manipulationor handling is necessary (e.g., no component or processing aid need beadded, as is required for traditional, two-part hydrosilylationcrosslinking systems), only proper storage. More particularly, thepresent disclosure provides for a composition whose polymer,crosslinker, and catalyst can be combined at manufacturing and stored inthat “ready-to-use” form. There is no need to separate the catalyst fromone or both of the crosslinker and/or polymer. Thus, the composition isused or dispensed at least about 8 hours, preferably at least about 1day, more preferably at least about a week, and even more preferably atleast about a month after all of its components were mixed together. Asa consequence of the foregoing, these “all-in-one” terpolysiloxanepastes are particularly useful in the additive manufacturing ofhigh-quality silicone rubber parts for applications requiring exposuresto temperatures as low as 100° C.

Additional advantages of the various embodiments of the invention willbe apparent to those skilled in the art upon review of the disclosureherein and the working examples below. It will be appreciated that thevarious embodiments described herein are not necessarily mutuallyexclusive unless otherwise indicated herein. For example, a featuredescribed or depicted in one embodiment may also be included in otherembodiments but is not necessarily included. Thus, the present inventionencompasses a variety of combinations and/or integrations of thespecific embodiments described herein.

As used herein, the phrase “and/or,” when used in a list of two or moreitems, means that any one of the listed items can be employed by itselfor any combination of two or more of the listed items can be employed.For example, if a composition is described as containing or excludingcomponents A, B, and/or C, the composition can contain or exclude Aalone; B alone; C alone; A and B in combination; A and C in combination;B and C in combination; or A, B, and C in combination.

The present description also uses numerical ranges to quantify certainparameters relating to various embodiments of the invention. It shouldbe understood that when numerical ranges are provided, such ranges areto be construed as providing literal support for claim limitations thatonly recite the lower value of the range as well as claim limitationsthat only recite the upper value of the range. For example, a disclosednumerical range of about 10 to about 100 provides literal support for aclaim reciting “greater than about 10” (with no upper bounds) and aclaim reciting “less than about 100” (with no lower bounds).

EXAMPLES

The following examples set forth methods in accordance with thedisclosure. It is to be understood, however, that these examples areprovided by way of illustration and nothing therein should be taken as alimitation upon the overall scope of the invention.

Example 1 Materials

The materials used in the following Examples were obtained from thesources described in this paragraph. Methylhydrido-co-dimethylsiloxanecrosslinker (“MeHS” HMS-151; MW=1800-2000; 15-18% MeHS; average Si-Hfunctionality of 4; trimethylsiloxy-terminated) was purchased fromGelest, Inc. (Morrisville, Pa.). BLUESIL™ THIXO ADD 22646 thixotropicadditive was obtained from Elkem Silicones (East Brunswick, N.J.).Hexamethyldisilazane-treated (trimethylsilylated) AEROSIL® R 812 S fumedsilica filler was purchased from Evonik (Olmsted, Ohio). Hydrosilylationcatalysts platinum(II) acetylacetonate (“Pt(AcAc)₂”) andtrimethyl(methylcyclopentadienyl)-platinum(IV) (“MeCpPtMe₃”) werepurchased from Sigma Aldrich (St. Louis, Mo.). Diethyl azodicarboxylate,DEAD, was purchased from Alfa Aesar (Ward Hill, Mass.), and1,3-dioxolane (99.5% stabilized) was purchased from Acros Organics(Pittsburgh, Pa.). All commercial materials were used as received.

Example 2 Polymer Synthesis

In the Examples that follow, three differentα,ω-dimethylvinylsilyl-terminated terpolysiloxanes (A, B and C ofReaction Scheme 1) were used as base polymers for paste preparations.The base polymers each included 0.3 mol % of methylvinylsiloxy-(“MeViS”) repeat units, varying amounts (y) of crystallizationdisrupting diphenylsiloxy- (“DiPhS”; 3.6 mol % in Polymer A),methylphenylsiloxy- (“MePhS”; 4.4 mol % in Polymer B), or diethylsiloxy-(“DiEtS”; 7 mol % in Polymer C), and the rest (i.e., 93-96 mol %,respectively) dimethylsiloxy- (“DiMeS”) repeat units. All threeterpolymers were prepared by anionic ring opening/equilibrationpolymerization of the corresponding mixtures of cyclosiloxanes, as shownin Reaction Scheme 1 and as described previously by:

-   -   iv. A. Zlatanic, D. Radojcic, X. Wan, J. M Messman, P. R.        Dvornic, Suppression of Crystallization in Polydimethylsiloxanes        and Chain Branching in Their Phenyl-containing Copolymers,        Macromol. 50 (2017) 3532-3543.    -   v. A. Zlatanic, D. Radojcic, X. Wan, J. M. Messman, P. R.        Dvornic, Monitoring of the Course of the Silanolate-Initiated        Polymerization of Cyclic Siloxanes. A Mechanism for the        Copolymerization of Dimethyl and Diphenyl Monomers, Macromol.        51 (2018) 895-905.    -   vi. A. Zlatanic, D. Radojcic, X. Wan, J. M. Messman, D. E.        Bowen, P. R. Dvornic, Dimethyl-Methylphenyl Copolysiloxanes by        Dimethylsilanolate-Initiated Ring Opening Polymerization.        Evidence for Linearity of the Resulting Polymer Structures. J.        Pol. Sci., Part A: Pol. Chem. 57(10) (2019) 1122-1129,        https://doi.org/1.002/pola.29367.

-   Each of (i)-(iii) above is incorporated by reference herein.    Selected characteristics of these terpolysiloxanes are shown in    Table 1.

TABLE 1 Selected Characteristics of Terpolysiloxanes A, B, and C ofReaction Scheme 1. IV_(t) ^(b), IV_(exp) ^(b), η^(d) at SEC-MALS-Visc.gI₂/100, gI₂/100, 25° C., dn/dc, Polymer ID M_(t) ^(a) DP_(t) g g f_(Vi)^(c) Pa · s mL/g M_(w) M_(n) A - 38,000 480 2.3 2.4 3.3 6.8 −0.07357,373 35,149 DiMeDiPh_(3.6)MeVi B - 30,500 400 3.0 3.5 3.6 4.7 −0.07946,321 29,283 DiMeMePh_(4.4)MeVi C - 36,900 480 2.4 2.6 2.9 4.2 −0.09445,636 28,355 DiMeDiEt₇MeVi Comonomer content MHS Polymer SEC-MALS-Visc.Parameters Feed, (by ¹H [η]^(f), R_(h), K, mol NMR), Polymer ID M_(p)PDI^(e) mL/g nm a mL/g % mol % A - 54,338 1.63 29.2 6.2 0.682 0.0178 3.64.0 DiMeDiPh_(3.6)MeVi B - 46,432 1.58 24.8 5.5 0.688 0.0160 4.4 4.7DiMeMePh_(4.4)MeVi C - 44,900 1.61 25.9 5.5 0.673 0.0203 7.0 6.7DiMeDiEt₇MeVi ^(a)M_(t): targeted molecular weight ^(b)IV_(t) andIV_(exp): iodine values theoretical and experimental, respectively^(c)f_(vi): Vi functionality of polysiloxane: f_(Vi) = M_(n)/EW_(Vi).The weight of Vi equivalent: EW_(Vi) = MW(I₂) × 100/IV = 25,380/IV^(d)η: dynamic viscosity of isolated polymer as determined by cone andplate viscometry ^(e)PDI: polydispersity index; PDI = M_(w)/M_(n)^(f)[η]: intrinsic viscosity as determined by Visco-Star II onlinedetector

Example 3 Paste Preparation

All pastes were compounded using a FlackTek DAC 150.1 FV SpeedMixer™, adual asymmetric centrifugal mixer, from FlackTek Inc., Landrum, S.C. Ina typical preparation, terpolymer base, crosslinker, thixotropicadditive and catalyst-inhibitor mutual solution were added to a mixingcup, followed by successive additions of trimethylsilylated AEROSIL® R812 S fumed silica filler in small portions. After addition of eachfiller increment the pastes were mixed for 20 sec at 3,000 rpm until thetotal desired amount of filler was added. Once the final portion offiller was added, the pastes were mixed a final time for 1 min at 3,000rpm. 50 g sample batches were compounded with filler in 10, 15, and 18wt. % of samples, respectively.

Unless otherwise stated below, the following quantities were utilized:

-   -   Thixotropic additive: 1.0% by weight (based on total        compositional weight);    -   Crosslinker: supplied at 1:1 [Si—H]:[Si-Vi];    -   Catalyst: 0.0125% by weight (based on total compositional        weight); and    -   Inhibitor: 0.25:1 ratio of DEAD:catalyst.

Example 4 Determination of Rheological Properties

Rheological studies of the pastes as described in Examples 5-7 wereperformed using an AR 2000ex rotational rheometer (TA Instruments, NewCastle, Del.) with a 40 mm 0° (parallel) plate geometry at a 1 mmgeometry gap, and all compounded pastes were subjected to oscillatoryand steady flow shear tests at 25° C. Samples were spread evenly ontothe center of the bottom (Peltier) plate, after which the upper platewas lowered to the established geometry gap to ensure the paste madecomplete contact with surfaces of both plates. Excess material thatseeped beyond the diameter of the top plate was scraped off anddiscarded. Oscillatory shear tests were performed at constant frequencyof 1 sec⁻¹, with the stress sweep ranging from 3 to 5,000 Pa. The yieldstresses of the pastes were determined from oscillatory stress valuesobtained from: (a) intersections of the storage (G′) and loss (G″)moduli, and (b) from 90% of the G′ plateau value. Steady state sheartests were performed in two stages with reciprocal linear shear ramps,the first ranging from 0.01 sec⁻¹ to 10 sec⁻¹ and the second from 10sec⁻¹ to 0.01 sec⁻¹. Thixotropy of the pastes was determined from theshear stress vs. shear rate plots obtained from the steady state sheartests by the presence of a hysteresis loop characteristic forthixotropic behavior. In some instances (as noted in the Figures)comparisons were made a commercially available thixotropic material soldby Dow Corning under the name Dowsil™ SE 1700.

Example 5 Effects of a Thixotropic Additive on Pastes

In order to achieve the desired thixotropic behavior of pastes, a seriesof preliminary tests was performed to evaluate the effectiveness ofthixotropic additive, BLUESIL™ THIXO ADD 22646 (from Elkem Silicones).This thixotropic additive is a comb-like copolymer (estimated M_(n) ofabout 45,000) having a polydimethylsiloxane main chain backbone andpoly(ethyl ether-co-propyl ether) pendant chains ending in eithercarbinol or methylether end-groups. Formulations containing 15 wt. % oftrimethylsilylated fumed silica filler (AEROSIL® R 812 S, EvonikIndustries), and 0, 1, 2 or 5 wt. % of BLUESIL™ THIXO ADD 22646 relativeto the terpolymer base were compounded. The results obtained for pastesprepared from MePhS-containing terpolymer (B) are shown in FIGS. 1 and2.

It can be seen from these data that the addition of only 1 wt. %BLUESIL™ THIXO ADD 22646 was sufficient to result in: (a) a significantincrease in storage modulus, from about 60 Pa (terpolymer and filleronly) to approximately 100,000 Pa; (b) a significant increase in yieldstress, from about 65 Pa to 1,900 Pa at 90% of G′ plateau value; and (c)a change from almost shear-thinning material to one with a broadthixotropic hysteresis loop. Interestingly, further increases inBLUESIL™ THIXO ADD 22646 content to 2 and 5 wt. %, respectively, did nothave a significant effect on the rheological properties of these pastes.As a consequence, 1 wt. % content was chosen as the optimal amount ofBLUESIL™ THIXO ADD 22646 for further paste formulations.

Example 6 Effects of Polymer Composition on Pastes

The flow behavior of pastes prepared from DiPhS-, MePhS- andDiEtS-containing terpolysiloxanes (A, B and C of Reaction Scheme 1,respectively) with 1 wt. % BLUESIL™ THIXO ADD 22646 and 15 wt. %AEROSIL® R 812 S filler are shown in FIGS. 3 and 4. This behaviorremained unchanged with time, as indicated by repeated weekly tests overa one-month period of storage at room temperature.

It can be seen from these FIGS. 3 and 4 that the change in terpolymertype (i.e., the change in crystallization disrupting repeat units inthese polymer compositions) had no effect on rheological properties oftheir pastes, which all showed very similar yield stresses of 300-350 Pa(if determined at 90% G′ plateau value), or 2,200-2,400 Pa (if G′/G″intersect was used).

Example 7 Effects of Filler Concentration on Pastes

The relative content of trimethylsilylated fumed silica filler AEROSIL®R 812 S was found to exert a substantial effect. FIGS. 5(A)-(C) and6(A)-(C) show results of oscillatory and steady shear tests conducted on“all-in-one” pastes containing 10, 15, or 18 wt. % of this filler,respectively.

The thixotropic behavior of pastes became more pronounced withincreasing amount of filler, producing hysteresis loops with greaterareas at higher levels of shear stress (see FIG. 6), but also enablingfine-tuning of their moduli by adding precise quantities of filler (seeFIG. 5). Thus, the yield stresses could be very precisely “dialed in”from 60-85 Pa, to 300-350 Pa, to 475-600 Pa (using the 90% G′ plateauvalue) or from 300-750 Pa, to 2,200-2,400 Pa, to 3,800-4,000 Pa (usingthe G′/G″ intersect), by increasing the filler content from 10 to 15 to18 wt. %, respectively (see Table 2).

TABLE 2 Yield stress values of pastes with varying levels of filler.Yield stress at G′/G″ Yield stress at Paste composition intersect, Pa90% G′, Pa SE 1700 1,400 430 A + 10% AS + 1% THX 540 75 B + 10% AS + 1%THX 380 60 C + 10% AS + 1% THX 750 85 A + 15% AS + 1% THX 2,200 300 B +15% AS + 1% THX 2,200 300 C + 15% AS + 1% THX 2,400 350 A + 18% AS + 1%THX 4,000 600 B + 18% AS + 1% THX 3,800 475 C + 18% AS + 1% THX 3,800475 All % values are in wt. % relative to the entire mass of thecomposition. AS: AEROSIL ® R 812 S THX: BLUESIL ™ THIXO ADD 22646

Example 8 UV-Activated Crosslinking and Shelf-Life Stability

UV-activated crosslinking by hydrosilylation of terpolysiloxanes A, B,and C used for preparation of the “all-in-one” pastes is depicted inReaction Scheme 2.

A dimethylsiloxy-co-methylhydridosiloxy copolymer (Gelest, Inc. HMS-151)containing declared 15-18 mol % Si—H side groups per molecule (averageSi—H functionality of 4) was used as a crosslinker. Platinumacetylacetonate, Pt(AcAc)2 andtrimethyl(methylcyclopentadienyl)-platinum (IV) (“MeCpPtMe3”) were usedas UV-activated hydrosilylation catalysts, and diethyl azodicarboxylate(“DEAD”) was used as a catalysis inhibitor in the absence of UVirradiation. The DEAD and catalyst were supplied in a preliminary seriesof experiments at a molar ratio [DEAD]:[catalyst] of 0.25:1. It wasverified that upon addition of crosslinker and catalyst/inhibitor, theoverall rheology of the resulting “all-in-one” pastes did not changewith respect to the results presented in the previous sections.

The stabilities of pastes were evaluated in darkness and in laboratorylight (i.e., exposed daily to standard laboratory light on a benchtop),and their propensity to crosslink was evaluated under a 400 W,irradiance at 3″=115 mW/cm², UV light by monitoring the onset ofgelation as the time when paste tackiness disappeared. The resultsobtained for pastes from terpolymers A, B, and C at different fillercontents (10, 15, or 18 wt. % AEROSIL® R 812 S), 1 wt. % BLUESIL™ THIXOADD 22646, and 125 ppm Pt(AcAc)₂ are shown in Table 3.

TABLE 3 UV-Activated Crosslinking and Shelf Stability of Pastes. 10 wt.% Filler 15 wt. % Filler 18 wt. % Filler A B C A B C A B C 400 W UV 9090 95 90 90 120 110 115 140 Lamp (sec) Daylight 10 6 5 (days) Dark(days) >40 ≈30 ≈30

Table 3 shows that pastes made from DiPhS- and MePhS-containingterpolymers (A and B, respectively) crosslinked slightly faster under UVirradiation with increasing filler content (from about 90 sec at 10 and15 wt. % filler to about 110-115 sec at 18 wt. %) than the pastes fromDiEtS-containing terpolymer (C), for which the crosslinking timesincreased from 95 to 120 to 140 sec for the same amounts of filler,respectively. The shelf-life stabilities of all pastes, however,displayed similar trends of behavior in both dark and in laboratorylight, seemingly diminishing as the amount of filler was increased,although no effect of polymer composition on these properties could bespecifically deciphered.

Effects of catalyst concentration and [DEAD]:[catalyst] ratio on therate of crosslinking under UV irradiation and on shelf-life stabilitiesin daylight and in the dark were then evaluated in more detail on pastesfrom MePhS-containing terpolymer C having 15 wt. % AEROSIL® R 812 S and1 wt. % BLUESIL™ THIXO ADD 22646. In this set of experiments, the[DEAD]:[catalyst] molar ratio was inverted to determine the change inUV-activated cure times and shelf stabilities when inhibitor was addedin molar excess relative to catalyst. Pastes with [DEAD]: [catalyst]ratios of 1:1, 2:1, and 4:1 molar were examined, and the resultsobtained for crosslinking onsets under 400 W UV irradiation are shown inFIGS. 7 and 8.

It can be seen from these FIGS. 7 and 8 that although both catalystsperformed effectively, MeCpPtMe₃ had shorter cure times for these pastesthan Pt(AcAc)₂. The fastest crosslinking times utilizing MeCpPtMe₃ wereobtained for its 1:1 molar combination with DEAD, reaching to as low as25 and 20 sec for the cases of 250 and 1,000 ppm catalystconcentrations, respectively. Most importantly, however, when the samepastes were kept in darkness or open to laboratory light on a bench top,they did not show any sign of crosslinking after 6 months and 6-9 days,respectively. In an extreme case (data not shown), the crosslinking timereached as short as 22 sec for 250 ppm catalyst concentration with noDEAD added. For this case, the corresponding shelf-life stability was 6months in darkness, and 4 days on the bench top, respectively. Furtherextrapolation of this data also indicated that cure times shorter than10 sec under the same experimental conditions with approximately 10,000ppm catalyst and at [DEAD]:[catalyst]=1:1 molar should be achievable.

Example 9 Printing

Pastes were prepared from DiPhS-, MePhS-, and DiEtS-containingterpolysiloxanes (A, B and C of Reaction Scheme 1, respectively),following the procedure described in Example 3. Each paste wasindividually tested by loading into opaque 30cc syringes andcentrifuging for 10 minutes at 4,000 rpm for degassing prior to DIWprinting. The shear thinning behavior of these pastes allowed forpneumatic or volumetric dispensing at a feature size (internal nozzlediameter) of 250 microns without issue. The samples were printed using aNordson Ultimus V pneumatic dispenser for extrusion at ˜65 psi pressurematched with a print speed of ˜15 mm/s to achieve the proper beaddiameter. Crosslinking was carried out in a UV chamber (Fusion UVSystems Series 300).

The thixotropy of the pastes was exemplified by excellent shaperetention following extrusion with consistent bead diameters observed,with all samples performing well. FIG. 9(A) shows a snapshot of aMePhS-containing terpolysiloxane sample during this DIW printingprocess. FIG. 9(B) is a photograph of a silicone rubber pad printed inthis Example, while FIG. 9(C) demonstrates the flexibility of thatprinted rubber pad. Finally, FIG. 9(D) is a cross-sectional view of theprinted rubber pad at 120× magnification, which verifies a consistentbead diameter was achieved. The as-printed samples held their respectiveshapes throughout each build prior to crosslinking via UV chamber.

1. A method of forming a three-dimensional structure, said methodcomprising one or more of the following: (i) curing a first compositionto form a first layer, wherein said first composition was entirelyformed at least 3 days prior to said curing (i) and comprises: at least50% by weight of a polymer that comprises the following monomers:

and a crystallization disruptor monomer comprising:

 where: each R₁ can be the same or different and is chosen from C₁ to C₆alkyls and C₁ to C₆ fluoroalkyls; R₂ is chosen from C₁ to C₆ alkyls;each R₃ can be the same or different and is chosen from phenyl, C₁ to C₆alkyls, and C₁ to C₆ fluoralkyls, wherein at least one R₃ is phenyl,ethyl, or a fluoroalkyl; the molar percentage of (I) in the polymer isfrom about 90% to about 98%; the molar percentage of (II) in the polymeris from about 0.1% to about 10%; and the molar percentage of (III) inthe polymer is from about 2% to about 10%; a hydrosilylationcrosslinker; a thixotropic additive; a catalyst a catalyst inhibitor;and a filler; (ii) curing a second composition to form a second layer onsaid first layer, wherein said first and second compositions can be thesame as or different from one another; and (iii) repeating (ii) one ormore times with further compositions that can be the same as the firstcomposition or different from the first composition so as to form one ormore additional layers, wherein (ii) or (iii) results in the formationof the three-dimensional structure.
 2. The method of claim 1, whereineach R₁ is methyl, R₂ is methyl, and (III) is chosen from one or moreof:


3. The method of claim 1, wherein: said hydrosilylation crosslinkercomprises a methylhydridosiloxane-dimethylsiloxane copolymer; saidcatalyst comprises a platinum catalyst; said catalyst inhibitorcomprises diethyl azodicarboxylate; said thixotropic additive comprisesa polydimethylsiloxane main chain backbone and poly(ethylether-co-propyl ether) pendant chains ending in carbinol end groups,methylether end groups, or both; and said filler comprises a silicafiller. 4-5. (canceled)
 6. The method of claim 1, wherein said curing(i) and curing (ii) comprise exposing said first and second compositionsto electromagnetic radiation so as to crosslink said polymer and formcured first and second layers.
 7. The method of claim 1, wherein saidfirst layer is formed on a support.
 8. The method of claim 1, whereinsaid curing (i) takes place at room temperature.
 9. The method of claim1, wherein said first composition is shelf-stable in a dark environmentfor at least 30 days and in ambient light for at least 4 days.
 10. Athree-dimensional structure formed according to the method of claim 11.A method of forming a three-dimensional structure, said methodcomprising one or more of the following: (i) curing a first compositionto form a first layer, wherein said first composition was entirelyformed at least 3 days prior to said curing (i) and comprises: at least50% by weight of a polymer that comprises the following monomers:

and a crystallization disruptor monomer chosen from one or more of:

 where: each R₁ can be the same or different and is chosen from C₁ to C₆alkyls and C₁ to C₆ fluoroalkyls; R₂ is chosen from C₁ to C₆ alkyls; themolar percentage of (I) in the polymer is from about 90% to about 98%;the molar percentage of (II) in the polymer is from about 0.1% to about10%; and the molar percentage of (III) in the polymer is from about 2%to about 10%; a hydrosilylation crosslinker; a transition metalhydrosilylation catalyst; and a thixotropic additive; (ii) curing asecond composition to form a second layer on said first layer, whereinsaid first and second compositions can be the same as or different fromone another; and (iii) repeating (ii) one or more times with furthercompositions that can be the same as the first composition or differentfrom the first composition so as to form one or more additional layers,wherein (ii) or (iii) results in the formation of the three-dimensionalstructure.
 12. The method of claim 11, wherein: said hydrosilylationcrosslinker comprises a methylhydridosiloxane-dimethylsiloxanecopolymer; said hydrosilylation catalyst comprises a platinum catalyst;and said thixotropic additive comprises a polydimethylsiloxane mainchain backbone and poly(ethyl ether-co-propyl ether) pendant chainsending in carbinol end groups, methylether end groups, or both. 13.(canceled)
 14. The method of claim 11, wherein said curing (i) andcuring (ii) comprise exposing said first and second compositions toelectromagnetic radiation so as to crosslink said polymer and form curedfirst and second layers.
 15. The method of claim 11, wherein said firstlayer is formed on a support.
 16. The method of claim 11, wherein saidcuring (i) takes place at room temperature.
 17. The method of claim 11,wherein said first composition is shelf-stable in a dark environment forat least 30 days and in ambient light for at least 4 days.
 18. Athree-dimensional structure formed according to the method of claim 11.